2.1 Psychologists Use the Scientific Method to Guide Their Research

Learning Objectives

  1. Describe the principles of the scientific method, and explain its importance in conducting and interpreting research.
  2. Differentiate hypotheses from predictions, and describe why operational definitions are important.

Psychologists are not the only people who seek to understand human behaviour and solve social problems. Philosophers, religious leaders, and politicians, among others, also strive to provide explanations for human behaviour. However, psychologists believe that research is the best tool for understanding human beings and their relationships with others. Rather than accepting the claim of a philosopher that people do, or do not, have free will, a psychologist would collect data to empirically test whether or not people are able to actively control their own behaviour. Rather than accepting a politician’s contention that creating, or abandoning, a new centre for mental health will improve the lives of individuals in the inner city, a psychologist would empirically assess the effects of receiving mental health treatment on the quality of life of the recipients. The statements made by psychologists are empirical, which means they are based on systematic collection and analysis of data.

The scientific method

All scientists, whether they are physicists, chemists, biologists, sociologists, or psychologists, use a common framework for conducting research that is called the scientific method. The scientific method is deceptively simple (see Figure 2.1); it can be distilled down to a series of steps.



This diagram shows the ongoing process of the scientific method, including making observations, thinking of interesting questions, formulating hypotheses, developing testable predictions, gathering data to test predictions, and developing general theories.
Figure 2.1. This displays an iterative approach to knowledge acquisition.

As psychologists learn more about something, that knowledge generates further questions that can be turned into hypotheses. As our knowledge is expanded, we may have to change theory to account for it.

In addition to requiring that science be empirical, the scientific method demands that the procedures used be objective, that is to say, free from the personal bias or emotions of the scientist. The scientific method proscribes how scientists collect and analyze data, how they draw conclusions from data, and how they share data with others. These rules increase objectivity by placing data under the scrutiny of other scientists and even the public at large. Because it is reported objectively, other scientists know exactly how the scientist collected and analyzed the data. This means that they do not have to rely only on the scientist’s own interpretation of the data; they may draw their own, potentially different, conclusions.

Most new research is designed to replicate — that is, to repeat, add to, or modify — previous research findings. The scientific method, therefore, results in an accumulation of scientific knowledge through the reporting of research and the addition to and modification of these reported findings by other scientists.

Laws and theories as organizing principles

One goal of research is to organize information into meaningful statements that can be applied in many situations. Principles that are so general as to apply to all situations in a given domain of inquiry are known as laws. There are well-known laws in the physical sciences, such as the law of gravity and the laws of thermodynamics, and there are a few universally accepted laws in psychology, such as the law of effect and Weber’s law. Due to the fact that laws are general principles of which the validity has already been well established, they are themselves rarely subjected to scientific testing directly.

The next step down from laws in the hierarchy of organizing principles is theory. A theory is an integrated set of principles that explains and predicts many, but not all, observed relationships within a given domain of inquiry. One example of an important theory in psychology is the stage theory of cognitive development proposed by the Swiss psychologist Jean Piaget (1952). The theory states that children pass through a series of cognitive stages as they grow, each of which must be mastered in succession before movement to the next cognitive stage can occur. This is an extremely useful theory in human development because it can be applied to many different content areas and can be tested in many different ways.

Good theories have four important characteristics. First, good theories are general, meaning they summarize many different outcomes. Second, they are parsimonious, meaning they provide the simplest possible account of those outcomes. The stage theory of cognitive development meets both of these requirements. It can account for developmental changes in behaviour across a wide variety of domains, and yet it does so parsimoniously by hypothesizing a simple set of cognitive stages. Third, good theories provide ideas for future research. The stage theory of cognitive development has been applied not only to learning about cognitive skills but also to the study of children’s moral (Kohlberg, 1966) and gender (Ruble & Martin, 1998) development.

Finally, good theories are falsifiable (Popper, 1959), which means the variables of interest can be adequately measured and the relationships between the variables that are predicted can be shown through research to be incorrect. If research findings show that your prediction was not supported, that is important information that you need to know. The stage theory of cognitive development is falsifiable because the stages of cognitive reasoning can be measured and because if research discovers, for instance, that children learn new tasks before they have reached the cognitive stage hypothesized to be required for that task, then the theory will be shown to be incorrect. Falsifiability is an important concept. Research findings would not be very useful if the results would only show if a prediction were right but not if it were wrong.

No single theory is able to account for all behaviour in all cases. Rather, theories are each limited in that they make accurate predictions in some situations, or just for some people, but not in other situations, or for other people. As a result, there is a constant exchange between theory and data. Existing theories are modified on the basis of collected data, the new modified theories then make new predictions that are tested by new data, and so forth. When a better theory is found, it will replace the old one. This is part of the accumulation of scientific knowledge.

Hypotheses and predictions

A hypothesis is a general statement about relationships between variables. Hypotheses are often generated by theories. An example of a hypothesis is “sleep is important for memory.” A hypothesis is general, but it can be tested in different ways depending on how you interpret it. One hypothesis can generate many specific predictions. For example, taking our previous hypothesis, we could generate the following three predictions:

  1. People who get more than seven hours of sleep will get a higher score on the ABC Memory Test than people who get less than seven hours.
  2. There is a positive association between amount of time students sleep and their grade point average.
  3. People suffering from insomnia show an increased ABC Memory Test score when they are successfully treated for insomnia.

The research prediction states the existence of a relationship between the specific variables of interest and the specific direction of that relationship. Psychologists use the term operational definition to refer to the measurement properties of a variable; operational definitions show exactly what is being measured. For example, we could refer to the variable of sleep in many different ways: self-reported number of hours of sleep, according to brainwaves measured in a sleep lab, the number of hours of sleep reported by a fitness tracking device worn by participants, and so on. We need a common understanding of what we mean by “sleep” (i.e., an operational definition). If we were measuring temperature, we would need to define what we mean by temperature: degrees Fahrenheit, degrees Celsius, or simply our best guess. The need for operational definitions is fundamental; if a variable is not defined precisely and specifically, then we can have no common understanding of how the variable was understood. Note that when we talk about the measurement properties of a variable, we are not talking about the experimental procedures for gathering data. An operational definition, in simplest terms, is the units in which something is measured.

The table below lists some potential operational definitions of variables that have been used in psychological research. As you read through this list, note that in contrast to the abstract conceptual variables, the operational definitions are very specific. This specificity is important for two reasons. First, more specific definitions mean that there is less danger that the collected data will be misunderstood by others. Second, specific definitions will enable future researchers to replicate the research.


Table 2.1. Examples of operational definitions
Conceptual Variable Operational Definitions
  • Number of presses of a button that administers shock to another student
  • Number of seconds taken to honk the horn at the car ahead after a stoplight turns green
Interpersonal attraction
  • Number of inches that an individual places his or her chair away from another person
  • Number of millimeters of pupil dilation when one person looks at another
Employee satisfaction
  • Number of days per month an employee shows up to work on time
  • Rating of job satisfaction from 1 (not at all satisfied) to 10 (extremely satisfied)
Decision-making skills
  • Number of groups able to correctly solve a group performance task
  • Number of seconds in which a person solves a problem
  • Number of negative words used in a creative story
  • Number of appointments made with a psychotherapist


Key Takeaways

  • Psychologists use the scientific method to generate, accumulate, and report scientific knowledge.
  • The goal of basic research is to extend our knowledge, while the goal of applied research is to find solutions to practical problems. Both types inform each other and work together to advance science.
  • Research reports describing scientific studies are published in scientific journals so that other scientists and laypersons may review the empirical findings.
  • Organizing principles, including laws, theories, and research hypotheses, give structure and uniformity to scientific methods.



Exercises and Critical Thinking

  1. Give an example from personal experience of how you or someone you know has benefited from the results of scientific research.
  2. Find and discuss a research project that in your opinion has ethical concerns. Explain why you find these concerns to be troubling.
  3. Later in this chapter we will examine the use of animals in research. Indicate your personal feelings about the use of animals in research. When, if ever, should animals be used? What principles have you used to come to these conclusions? Review your answers after reading the rest of Chapter 2.

Image Attributions

Figure 2.1. Scientific Method 3 by Whatiguana is used under a CC BY-SA 4.0 license.


Kohlberg, L. (1966). A cognitive-developmental analysis of children’s sex-role concepts and attitudes. In E. E. Maccoby (Ed.), The development of sex differences (pp. 82–173). Stanford, CA: Stanford University Press.

Piaget, J. (1952). The origins of intelligence in children (M. Cook. Trans.). New York, NY: Norton.

Popper, K. R. (1959). The logic of scientific discovery. New York, NY: Basic Books.

Ruble, D., & Martin, C. (1998). Gender development. In W. Damon (Ed.), Handbook of child psychology (5th ed., pp. 933–1016). New York, NY: Wiley.


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Psychology - 1st Canadian Edition Copyright © 2020 by Sally Walters is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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